Resin processing equipment such as extruders and those used for injection molding is generally run for long periods of time. During these periods, residual materials can build up on various parts of the equipment such as the screw, barrel, or die. This material buildup can result from the carbonization or decomposition of resin or plastic product due to high processing temperatures or from the adherence of some of the resin or plastic product to the equipment surfaces for much the same reason. Cleaning is periodically required to remove these materials from the equipment in order to prevent contamination of the finished product.
In addition to the problem raised by carbonized and decomposed material, a problem is raised by the fact that extrusion and/or injection molding equipment is seldom utilized exclusively for one specific resin. Rather, it is used to process a variety of polymers. It is, therefore, imperative to flush or purge the resin presently being processed before a different polymer is introduced into the extruder, injection molding apparatus, or other equipment of a similar nature.
Typical cleaning methods include disassembly to get at the screw, die, or other parts, followed by physical removal of the objectionable material, which has adhered to the metal surfaces. These methods are time consuming and expensive, and require reheating of the equipment for the next run after it is reassembled. Another typical cleaning method is to purge the residual material with large quantities of the resin to be processed in the next run. This technique is, again, time consuming and expensive. Further, it does not insure that all of the carbonized and/or decomposed or other material is removed from the metal surfaces.
Acrylic resins have been used as purge materials; however, their use requires relatively higher processing temperatures due to their relatively high melting points. This not only increases energy consumption, but also the tendency for the material to be cleaned out to carbonize or decompose due to the increased heat. Subsequent removal of the acrylic resin is difficult especially when the machine temperature has to be lowered for the next material to be processed. Contamination and/or clogging of the screen pack or breaker plate by unmelted acrylic resin is likely under these circumstances.
Fillers such as talc, calcium carbonate, or mica have been added to polymer based purge compositions. These fillers, however, have relatively low Mohs hardness values in the 1 to 2 range. These low values do not provide good scouring action because the soft particles have a tendency to crush or break apart when pushed against the walls of an extruder or molding barrel. A Mohs hardness of 3 or more would be advantageous.